Well to make it short, the A400 is actually not built in Seville, Seville is just the place where the final assembly of the aircraft takes place. The “building” of the aircraft involves far more than just one country and has a pretty crazy supply chain which requires heavy transportation measures, an excellent timing, a good communication between all partners and a well-matched logistical operation.
First let’s answer the question of where the A400 is being built.
The manufacturing process of the A400 is conducted by six countries in total. These are, Germany, France, Belgium, Turkey, the U.K. and last but not least Spain. The exact parts for which each country is responsible are shown in the picture below.
Basically the process “starts” in Turkey, Hamburg and the U.K and once the parts that are build there are finished they are transported to the next stop where they are needed to be joined together with other parts. These newly combined parts, e.g., the complete fuselage, are then again transported to the next stop until all finished parts reach Seville and are all added together in order to assembly and finish the plane. You can see the path of the parts in the illustration below.
This picture also shows the means of transportation that is needed to haul the big pieces in a fast way, the Beluga transportation plane which itself is pretty crazy but effective design.
Like I said in the beginning, the manufacturing of the A400 is done with a pretty crazy supply chain that definitely requires a very good coordination. If you would imagine that one step in the supply chain has delays and problems, the directly following steps are also impacted and in the end the whole production will take longer than planned.
To be honest, for me this concept of having so many parties involved in the production of one aircraft is a unnecessary complication that only begs for problems to happen. But who am I to judge, they probably had pretty capable people come up with that plan and thought things through.
(Source for all information and pictures: https://extendaplus.es/aeroespacial/files/2013/05/1.Alberto-Guti%C3%A9rrez-Industrial-aspects-and-GSC-A400M-Programme-23-May-2013-v-final_Alberto-Gutierrez.pdf)